domain without www doesn't work

Hi
i have installed ispconfig 2.3.3-dev (that supports mailman) and hosted my other domain as a virtual host but it works with www like www.mydomain.com but without www when i enter mydomain.com in browser it doesn't work and get the following message on screen:

Shared IP
This IP address is shared. For access to the web site which you look for, enter its address instead of its IP.
For questions or problems please contact the server administrator.

i don't know the meaning of Hostname and Domain in ispconfig control panel. www means hostname?